Multiple Machines
Overview
Well ok - this one is really a bit of a captain obvious choice, but there are things you can do to make this more comfortable for your user. By multiple machines I mean the user has two or more computers on which they work almost simultaneously.
Performance
This is easily the best performing option of the lot because you have a physical machine for each operating system you're running. Everything is running natively and the user has direct access to the machines.
Ease of Use
This is where things can get a bit sticky with this option. The user is continually switching between computers and possibly keyboards and mice. A traditional option for this is a kvm, which is an improvement, but I feel there is a better solution. Each machine has one (or more) monitors attached to it. Place these monitors as you would physically for a multiple monitor machine and install Synergy on all the machines.
Ease of Configuration
On the machine with the most comfortable access to a keyboard and mouse for the user run Synergy in server mode. On the other machines set it up as a client. See for details on how to configure it.
The wacom will trigger the switch to another screen on the edges, but won't allow you to work on another screen unfortunately. You may wish to look at using a kvm for artists who wish to have a wacom switch between multiple machines and only connecting the wacom into the kvm.
Flexibility
This option allows you to run any software you like so has a great deal of flexibility in that regards, but provides no additional benefits.
Cost
This is the most expensive option as it requires multiple machines for every artist who needs software across multiple operating systems. Synergy is free software released under the GPL.